[gcc r14-9002] libstdc++: Use memset to optimize std::bitset::set() [PR113807]

[gcc r14-9002] libstdc++: Use memset to optimize std::bitset::set() [PR113807]

[Jonathan Wakely]

https://gcc.gnu.org/g:e7ae13a858f36031b8fd3aa07362752ff2b19b2e

commit r14-9002-ge7ae13a858f36031b8fd3aa07362752ff2b19b2e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 8 15:46:08 2024 +0000

    libstdc++: Use memset to optimize std::bitset::set() [PR113807]
    
    As pointed out in the PR we already do this for reset().
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/113807
            * include/std/bitset (bitset::set()): Use memset instead of a
            loop over the individual words.

Diff:
---
 libstdc++-v3/include/std/bitset | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/bitset b/libstdc++-v3/include/std/bitset
index 3243c6497314..16c4040f532f 100644
--- a/libstdc++-v3/include/std/bitset
+++ b/libstdc++-v3/include/std/bitset
@@ -177,8 +177,15 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       _GLIBCXX14_CONSTEXPR void
       _M_do_set() _GLIBCXX_NOEXCEPT
       {
-	for (size_t __i = 0; __i < _Nw; __i++)
-	  _M_w[__i] = ~static_cast<_WordT>(0);
+#if __cplusplus >= 201402L
+	if (__builtin_is_constant_evaluated())
+	  {
+	    for (_WordT& __w : _M_w)
+	      __w = ~static_cast<_WordT>(0);;
+	    return;
+	  }
+#endif
+	__builtin_memset(_M_w, 0xFF, _Nw * sizeof(_WordT));
       }
 
       _GLIBCXX14_CONSTEXPR void